A structure of a conventional controller for an induction motor is shown in FIG. 1, wherein numeral 1 denotes an induction motor, numeral 21 denotes a transistor inverter circuit for driving the induction motor 1 by a variable frequency, numeral 22 denotes a frequency command generator, numeral 23 denotes a function generator, numeral 24 denotes a primary voltage command generating circuit, and numeral 25 denotes a PWM circuit.
The theory of the frequency control by the controller in the induction motor will be described as follows. Equivalent T circuit in one phase in the conventional controller for an induction motor is shown in FIG. 2, wherein R.sub.1 denotes a primary resistance, R.sub.2 denotes a secondary resistance, l.sub.1 denotes a primary leakage inductance, l.sub.2 denotes a secondary leakage inductance, M denotes a primary/secondary mutual inductance, .omega..sub.l denotes a primary frequency, .omega..sub.s denotes a slide frequency, V.sub.1 denotes a primary voltage, E.sub.0 denotes a clearance induced voltage, I.sub.1 denotes a primary electric current, and I.sub.2 denotes a secondary electric current.
Clearance magnetic flux .PHI..sub.0 is fixed from the induced voltage E.sub.0 and the primary frequency .omega..sub.1. Time integral of voltage is magnetic flux. Accordingly, an expression (1) is established. EQU .PHI..sub.0 .dbd.E.sub.0 /.omega..sub.1 ( 1)
Electric current I.sub.2r which generates torque, acting on the magnetic flux .PHI..sub.0, is the same phase component as an effective part of the secondary electric current I.sub.2, i.e., the induced voltage E.sub.0. Accordingly, I.sub.2r is fixed by an expression (2) as shown in FIG. 2. ##EQU1##
Generated torque T.sub.e in the induction motor is proportional to the product of the magnetic flux .PHI..sub.0 and the electric current I.sub.2r. Accordingly, an expression (3) is established. EQU T.sub.e .dbd.K.PHI..sub.0 I.sub.2r ( 3)
K: Propotional Constant PA1 an induction motor, PA1 a current detecting means for detecting a primary current of the induction motor, PA1 a variable-frequency power converting means for driving the induction motor by a variable-frequency, PA1 a no-load voltage processing means for outputting a no-load voltage command value of the induction motor by inputting a primary frequency command value and an exciting current command value, PA1 an error current component processing means for processing an error current component which becomes zero when an actual value of a primary flux occurring inside the induction motor, by inputting the primary current, the primary frequency command value, and the exciting current command value, coincides with a set value of the primary flux obtained from a product of the exciting current command value and a primary self-inductance of the induction motor, PA1 a compensation voltage processing means for processing a compensation voltage to direct the error current component value to zero by inputting the primary frequency command value and the output from the error current component processing means, and PA1 a primary voltage command processing means for processing a primary voltage command value of the induction motor by inputting the primary frequency command value, the no-load voltage command value, and the compensation voltage, and for outputting the primary voltage command value to the variable-frequency power converting means. PA1 an induction motor, PA1 a current detecting means for detecting a primary current of the induction motor, PA1 a variable-frequency power converting means for driving the induction motor by a variable-frequency, PA1 a no-load voltage processing means for outputting a no-load voltage command value of the induction motor by inputting the primary frequency command value and an exciting current command value, PA1 an error current component processing means for processing an error current component which becomes zero when an actual value of a primary flux occurring inside the induction motor, by inputting the primary current, the primary frequency command value, and the exciting current command value, coincides with a set value of the primary flux obtained from a product of the exciting current command value and a primary self-inductance of the induction motor, PA1 primary resistance compensating means for processing a compensation quantity of a primary resistance set value by inputting the output from the error current component processing means, PA1 a compensation voltage processing means for processing a compensation voltage to direct the error current component value to zero by inputting the primary frequency command value, the output from the error current component processing means, and the output from the primary resistance compensating means, and PA1 a primary voltage command processing means for processing a primary voltage command value of the induction motor by inputting the primary frequency command value, the no-load voltage command value, and the compensation voltage, and for outputting the primary voltage command value to the variable-frequency power converting means. PA1 a variable-frequency power converting means for driving the induction motor by a variable-frequency, PA1 a no-load voltage processing means for outputting a no-load voltage command value of the induction motor by inputting a primary frequency command value, a primary frequency compensation value, and an exciting current command value, PA1 an error current component processing means for processing an error current component which becomes zero when an orthogonal component on a rotating co-ordinate axis rotated by the primary frequency of the primary current and an actual value of a primary flux occurring inside the induction motor, by inputting the sum of the primary frequency command value and the frequency compensation value, the primary current, and the exciting current command value, coincide with a set value of a primary flux obtained from a product of the exciting current command value and a primary self-inductance in the induction motor, PA1 a compensation voltage processing means for processing a compensation voltage to direct the error current component value to zero by inputting the sum of the primary frequency command value and the primary frequency compensation value, and the output from the error current component processing means, PA1 a primary voltage command processing means for processing a primary voltage command value in the induction motor, by inputting the sum of the primary frequency command value and the primary frequency compensation value, and the no-load voltage command value and the compensation voltage, and for outputting the primary voltage command value to the variable-frequency power converting means, and PA1 a torque limiting means for processing the primary frequency compensation value for the generated torque in the induction motor not to be over a limit value by inputting the output from the error current component processing means. PA1 an induction motor, PA1 a current detecting means for detecting a primary current of the induction motor, PA1 a variable-frequency power converting means for driving the induction motor by a variable-frequency, PA1 a no-load voltage processing means for outputting a no-load voltage command value of the induction motor by inputting a primary frequency command value and an exciting current command value, PA1 an error current component processing means for processing an error current component which becomes zero when an orthogonal component on a rotating co-ordinate axis rotated by the primary frequency of the primary current and an actual value of a primary flux occurring inside the induction motor, by inputting the primary frequency command value, the primary current, and the exciting current command value, coincide with a set value of a primary flux obtained from a product of the exciting current command value and a primary self-inductance in the induction motor, PA1 a compensation voltage processing means for processing a compensation voltage to direct the error current component value to zero by inputting the primary frequency command value and the output from the error current component processing means, PA1 a primary voltage command processing means for processing a primary voltage command value of the induction motor by inputting the primary frequency command value, the no-load voltage command value, and the compensation voltage, and for outputting the primary voltage command value to the variable-frequency power converting means, and PA1 a torque controlling means for processing the primary frequency command value for the generated torque in the induction motor to comply with the command value by inputting the output from the error current component processing means.
The expressions (1) and (2) are put in the expression (3), which establishes an expression (4). ##EQU2##
In the expression (4), when E.sub.0 /.omega..sub.1 is controlled to be fixed, the generated torque T.sub.e varies, depending upon the slide frequency .omega..sub.s. The maximum torque T.sub.max is obtained by integrating the expression (4) by the slide frequency .omega..sub.s and making its numerator 0, which establishes an expression (5). ##EQU3##
Accordingly, the maximum torque T.sub.max has no relationship with variation of .omega..sub.1, when E.sub.0 /.omega..sub.1 is fixed.
Since the induced voltage E.sub.0 can not be easily detected in fact, it is typical that V/F constant controlling system is used, in which the primary voltage V.sub.1 is proportional to .omega..sub.1 and the value of V.sub.1 /.omega..sub.1 is controlled to be fixed.
In this case, in the area where the primary frequency .omega..sub.1 is low, potential drop by the primary resistance R.sub.1 occurs on the primary voltage V.sub.1. Accordingly, V.sub.1 is previously amplified corresponding to R.sub.1 and I.sub.1 in the low voltage area.
An operation of the controller shown in FIG. 1 will be described as follows. The function generator 23 inputs a primary frequency command .omega..sub.1 * outputted from the frequency command generator 22 in accordance with the function relationship shown by the real line in FIG. 3, and outputs an amplitude command V.sub.1 * of the primary voltage.
The primary voltage command generating circuit 24 outputs primary voltage commands V.sub.1u *, V.sub.1v *, and V.sub.1w * to be impressed on each primary coil winding of the induction motor 1, after an expression (6) is operated in accordance with the amplitude command V.sub.1 * of the primary voltage and the primary frequency command .omega..sub.1 *. ##EQU4##
The PWM circuit 25 generates a base signal which controls an ON/OFF action of a transistor (not shown in the drawings) comprising the transistor inverter circuit 21 corresponding to the primary voltage commands V.sub.1u *, V.sub.1v *, and V.sub.1w *. As a result, the primary voltages V.sub.1u, V.sub.1v, and V.sub.1w actually impressed on the induction motor 1 are controlled to comply with each command. Accordingly, it is possible to control a frequency of the induction motor 1, i.e., a rotary speed, corresponding to the primary frequency command .omega..sub.1 *.
In the conventional controller for an induction motor constructed as above, when a large amount of the generated torque is necessary in a low speed revolution, the primary voltage command V.sub.1 * must be previously set to a high value to compensate for a voltage dropped by the primary resistance R.sub.1, as shown in FIG. 3.
However, it is difficult to compensate precisely for the dropped voltage, since a value of the primary resistance R.sub.1 is fluctuated by temperature. Accordingly, when the voltage compensated is smaller than the voltage actually dropped, on the assumption that a load torque is steadily impressed on the induction motor, the induction motor can not be started because of a lack of the generated torque in a low speed revolution. In contrast, when the voltage compensated is larger than the voltage actually dropped, an operation of the inverter circuit must be stopped to protect the inverter circuit from an excess current by the large amount of the primary current in a low speed revolution. When a machine operated by the induction motor is different, the amount of the generated torque being the same, a total moment of inertia is different. Accordingly, a variation rate in a rotary speed of the induction motor is different. Accordingly, unless the variation rate of the primary frequency command .omega..sub.1 * is properly adjusted, the induction motor speed is not adjusted in accordance with .omega..sub.1 *, and the operation of the inverter circuit must be stopped to protect the inverter circuit from the excess current by the large amount of primary current.
When a sudden impact load is impressed, the operation of the inverter circuit must be stopped to protect the inverter circuit from the excess current by the large amount of primary current because of a lack of a limiting function for the torque in the conventional controller for an induction motor.
In the conventional controller for an induction motor, the primary voltage previously set according to the value of the primary frequency command is outputted as above, and there is no function for controlling the torque generated in the induction motor. Accordingly, the torque control is impossible in principle.